Embodiments disclosed herein relate to solder masks such as those employed in the manufacture of printed circuit boards. In particular, embodiments disclosed herein relate to solder mask inks having suitable viscosity for aerosol jet printer application.
Printed circuit boards (PCBs) or printed wiring boards (PWBs) (hereinafter collectively PCB's) are platforms that connect and interface electronic components with each other and with other elements in computers, communication devices, consumer electronics, automated manufacturing and inspection equipment. PCB's may be produced from a base substrate, typically an insulating material, on which a thin copper layer is laminated or plated. Chemical etching is then used to remove areas of the copper to produce electrically conducting paths or traces. The traces permit electrical interconnection of the components attached to the PCB.
An insulative material, referred to as a solder mask, is then applied over the copper conducting paths. Solder masks protect the conducting paths on the PCB from being coated with solder during soldering steps, while leaving uncovered only the conducting pads that need to be contacted with molten solder. The solder mask layer on simple PCBs may be produced using screen-printing or spin-casting techniques. However, more densely populated PCBs typically utilize lithographic techniques to form a patterned solder mask on the copper layer.
Lithographic techniques used to prepare solder masks can involve multi-step sequences that are material and energy-intensive. For example, the process usually involves film coating, lithography, wet etching and curing, as indicated in the flow diagram of FIG. 1. In such a process, the solder masks are often epoxy-based materials that are spin-coated or applied in an analog fashion, followed by subtractive etching. The process tends to deteriorate the chemical and physical resistance of the final cured solder mask. Because the first step of photolithography is non-selective, via holes in PCB are often partially or completely filled with solder mask. Removing the solder mask for high aspect ratio via holes is a very difficult and sometimes impossible task. Ultimately, such methods can be a costly and wasteful.
Although digital methods for depositing solder masks is desirable attempts at inkjet printing, for example, have run up against limitations due to the very low viscosity requirements (less than about 20 cps). By contrast, commercial solder mask resists typically have very high viscosities (greater than about 10,000 cps), and thus, inkjet printing solder mask systems are difficult to put into practice.
Alternatively, screen printing has been developed for depositing solder mask resist. Although it can overcome the via hole plugging issue, screen printing often yields solder masks with low resolution and poor registration. In addition, screen printing requires a flat surface. PCBs with relief structures on the surface are not amenable to screen print solder mask thereon. Similarly, solder masks generally cannot be screen printed on curved surfaces or on 3D electronics.